This invention pertains to the valve art and more particularly to a diaphragm valve. The invention is particularly applicable to a fluid system requiring high purity, good fluid lead integrity, low internal volume and high reliability. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other environments and applications.
Certain sectors of the manufacturing industry have imposed increasingly stricter requirements on valve manufacturers, some of these standards being unthinkable only a few years ago. In addition to providing reliable open and closed positions to regulate fluid flow, the industry has become even more concerned with other characteristics of the valve, particularly cleanliness both before, during and after actuation. With more sensitive and accurate sensing equipment and the increased demand for purity in fluid systems, valve manufacturers are ever conscious of new and different arrangements that satisfy their customer's needs.
As will be appreciated, plastic has been incorporated into valve designs due to the reliable sealing, durability, adaptability, and long cycle life attainable with selected plastics in the valve environment. Unfortunately, the use of plastic has come under increased scrutiny due to its ability to entrap and outgas fluids, i.e. release of fluid to the system at an undesired time, that could contaminate the system.
In a related manner, use of plastic can also have a dramatic effect on the time required to clean or purge the system. Since the plastic can entrap fluid, longer cleaning times are necessary which results in increased down-time for the fluid system. Because of the tremendous cost to the manufacturer, decreasing down-time is always desirable and limiting the amount of plastic in the valve design is a key goal.
Other valve designs incorporate greater amounts of plastic into the valve seat: either (i) by the method of containing the seat in the valve body, or (ii) by enclosing the seat in a movable valve member. The excessive surface area and volume of the plastic used in the valve seats of these designs contributes to fluid entrapment and subsequent potential system contamination.
Many present day valves include multi-layer diaphragm constructions. This type of design has a primary drawback. That is, if one or more layers of the composite diaphragm fails, the operator may be unaware of its failure. This could cause a virtual leak between the diaphragm layers which would serve as an area for potential fluid entrapment. The fluid could be trapped between the diaphragm layers and released to the system at an undesired time.
The number of diaphragms in the valve can also cause problems. Too few layers in a multi-layer design may not adequately address pressure containment concerns. On the other hand, too many layers makes the multi-layer diaphragm assembly too stiff and potentially unable to be moved to effect valve closure. Single layer diaphragm designs can also be subject to the inadvertent assembly of more than one diaphragm without operator awareness.
Reliability of operation is always a primary concern with valves handling dangerous fluids. Under the general category of reliability, valve manufacturers have employed various arrangements to assure that the valve will open under all conditions. One arrangement is to structurally tie the actuating stem to the seat by forming an opening through a central portion of the diaphragm and directly fastening the stem and plastic valve seat together. This arrangement, where the plastic is secured to the diaphragm rather than the valve body, compromises the integrity of the diaphragm and defines yet another potential leak path that must be effectively addressed by the valve design.
An alternate arrangement secures one face of the diaphragm to the actuating stem by welding or the like. Although the integrity of the diaphragm is maintained, this alternate arrangement has detrimental aspects of its own related to the welding operation.
With some constructions it is even possible to deflect the diaphragm to a position where it can't be operated. For example, an overcenter position of the diaphragm may occur so that movement of the diaphragm from a first position toward a second position may be precluded even though the actuating stem is still operational.
Still another aspect of reliability is effective sealing or closure of the valve. A metal seal would solve the entrapment problem but creates other problems. The force required to make a seal between a metal diaphragm and a metal valve seat is substantial. This is particularly a problem when an air actuated version of the valve is desired. That is, a manual actuator can easily supply a large closing force, but the closing force of the air operated version is dictated by the pressure source available to actuate the valve and also size constraints on the actuator itself. Even then, highly polished metal seal surfaces do not have the ability to conform to different operating conditions, in contrast to valves employing the plastic seats. Thus, the (i) metal-to metal and (ii) plastic valve seat designs are directed to entirely different structures that have their respective attributes and deficiencies.
Another major concern with the air actuated version of the valve design is reducing the actuating force, and thus the actuating pressure, required to operate the valve, i.e., move the valve member toward open and closed positions. Multiple layers as in the composite diaphragm arrangement add to the stiffness of the diaphragm. This requires increased pressure to actuate the valve. Since the source of pressure is oftentimes located at remote locations relative to the valve, it becomes necessary to increase the actuating system pressure due to the pressure loss in delivering the gas to the point of use. This, in turn, encounters unnecessary expense.